Analytical electron microscopy is frequently used to investigate the chemical composition of materials down to sub-nanometer length scales. Electron energy loss spectroscopy (“EELS”) relies on measurements of the energy distribution of incident electrons transmitted through the microscope sample, while energy dispersive x-ray spectroscopy (“EDS”) relies on measurements of the energy distribution of x-rays emitted from regions of the sample exposed to the incident electron beam. Though these two techniques originate in inelastic scattering phenomena, the processes by which the incident high energy electron beam excites bound state electrons within the sample, they differ in their ease of applicability to different elements. Generally, EELS is better suited to lighter elements—corresponding to characteristic energy losses up to about 3 keV—while EDS is better suited to heavier elements—corresponding to characteristic x-ray energies up to about 30 keV. Because EELS relies on transmitted electrons, it is used almost exclusively in transmission electron microscopy (“TEM”) and scanning transmission electron microscopy (“STEM”). Because EDS does not rely on transmitted electrons, it can be used in TEM and STEM, and also in scanning electron microscopy (“SEM”), which does not require electron transparent samples.
In EELS and EDS, quantitative chemical information is obtained by measuring the intensity of the characteristic peaks (also called “edges” in EELS, in recognition of their distinctive shape) associated with particular elements, which are often observed against a highly non-uniform background. The strength of signal associated with a given peak or peaks, and thus the accuracy and sensitivity of the measurement of the concentration of a given element, can be affected by a range of factors, including local sample composition and morphology, the spatial/energy profile and coherence of the incident beam, and the resolution and other characteristics of the X-ray or EELS detection system, that, taken together, can be extremely difficult to quantify.
When investigating samples comprising regions of crystalline material, the incident beam is frequently aligned close to a high symmetry crystallographic direction (also referred to as a “zone axis”). This can be deliberate—for example, when obtaining atomic resolution images in TEM/STEM or seeking information about crystallographic defects, such as grain boundaries or interfaces—or can occur accidentally, particularly when investigating polycrystalline materials comprised of crystallites having a range of different orientations. In any event, when the incident beam is aligned close to a zone axis, the coherent scattering by the periodic potential of the crystal (so-called elastic scattering) can strongly affect the peak intensities measured in EELS or EDS. In some circumstances, these “channeling” effects can be exploited to provide information about the location, on an atomic scale, of chemical species within a crystal structure. See, e.g., S. Van Aert et al., Electron Channeling Based Crystallography, 107 Ultramicroscopy 551-58 (2007). In general, however, channeling makes it more difficult to extract meaningful compositional information from crystalline samples. For example, a change of only one degree in the orientation of the incident electron beam can lead to changes of as much as 20% in the apparent relative composition of two elements, as measured by the strength of the characteristic x-ray signal. See, e.g., Frederick Meisenkothen et al., Electron Channeling: A Problem for X-Ray Microanalysis in Materials Science, 15 Microscopy and Microanalysis 83-92 (2009). Having a beam orientation close to a zone axis can also reduce the overall intensity of EELS and EDS peaks relative to the background, increasing the stochastic noise and the uncertainty of quantitative measurements.
Elemental composition maps can be generated by performing quantitative EELS or EDS peak measurements at multiple sample locations, assuming the difficulties noted above, associated with spectrum acquisition from discrete locations, can be overcome. When generating compositional maps, further difficulties are associated with the need to move the beam between different locations while maintaining consistent data collection conditions over extended time periods. These difficulties include sample degradation and drift and changes in signal intensity arising from electron optical, mechanical and electronic instabilities. Further, the changes in EELS or EDS peak intensities from crystalline samples associated with small changes in the angle of the incident beam are of particular concern for compositional mapping, since the relative angle of the incident beam can be affected as the beam is moved between locations by changes in electron optical conditions or sample morphology. Such changes in relative incident beam angle with location are almost inevitable in studies of polycrystalline materials, which, as noted above, generally contain a range of differently orientated crystallites.
Unlike the peaks used for quantitative compositional analysis in EELS and EDS, which originate from the interaction of the high energy electron beam with electrons bound to individual atoms, quantitative structural analysis by electron diffraction relies on measurements of the intensities of diffracted beams that originate from the interaction of high energy electron beam with the periodic potential of the crystal. Like the peaks in EDS and EELS, the intensity of these diffracted beams can be strongly affected by a range of factors that are not always easily quantifiable, including local sample composition and morphology, and variations in the spatial/energy profile and coherence of the incident beam. Further, when the incident beam is aligned along a high symmetry crystallographic orientation, as is generally required for meaningful structural analysis, intensity is dynamically redistributed between the incident and multiple diffracted beams in a complicated and not readily quantifiable manner.
Structural analysis of crystalline materials using transmission electron diffraction can be facilitated by applying a modification of the Buerger x-ray diffraction technique known as precession electron diffraction (“PED”), in which the incident electron beam (normally aligned with the optical axis of the electron microscope) is inclined away from and rotated (“precessed”) around a high symmetry crystallographic direction of the fixed TEM sample, and in which transmitted beams are de-scanned using a complimentary precession algorithm to re-align them with the optical axis of the electron microscope. See R. Vincent & P. A. Midgley, Double Conical Beam-rocking System for Measurement of Integrated Electron Diffraction Intensities, 53 Ultramicroscopy 271-82 (1994). PED can be qualitatively understood as suppressing the “dynamical” redistribution of intensity between incident and multiple diffracted beams associated with high symmetry directions, thereby approaching the more easily modeled “quasi-kinematical” conditions. However, a full theoretical understanding of the factors that govern the transition from dynamical to quasi-kinematical conditions in PED remains elusive. See E. Mugnaioli et al., “Ab Initio” Structure Solution from Electron Diffraction Data Obtained by a Combination of Automated Electron Tomography and Precession Technique, 109 Ultramicroscopy 758-65 (2009); T. A. White et al., Is Precession Electron Diffraction Kinematical? [Parts I and II], 110 Ultramicroscopy 763-770 (2010).
For typical TEM accelerating voltages, a relatively large precession angle, typically from 1-3 degrees, is required to suppress dynamical scattering enough to implement PED. The technique has been further developed as a means to facilitate the analysis of complicated phases, including by a combination of PED with x-ray and neutron diffraction data. See EP 1 665 321 B1. A beam scanning protocol analogous to PED—also aimed at achieving quasi-kinematical conditions to facilitate structural analysis—has also been developed, but using an oscillatory or pendulum-like motion (“EDPM”) in place of precession. See WO/2008/060237. Other work has applied a beam scanning protocol and transmission electron diffraction to facilitate the acquisition of orientation and structural phase maps (see WO 2010/052289), and to suppress spurious diffraction contrast in TEM images of the same crystallographic feature obtained along different directions. See J. M. Rebled et al., A New Approach to 3D Reconstruction from Bright Field Tem Imaging: Beam Precession Assisted Electron Tomography, 111 Ultramicroscopy 1504-11 (2011).
The present invention alleviates many of the difficulties of quantitative EDS and EELS measurements of samples containing crystalline or polycrystalline regions by applying a beam scanning protocol to maximize signal strength and diminish spurious signal variations associated with changes in relative incident beam angle, thereby allowing improved compositional mapping of samples containing crystalline or polycrystalline regions by STEM, TEM and SEM.